Sold my co-sleeper today. I got a bit choked up when it left. Not because I wanted to hold onto it. Just because it represented that I really am saying goodbye to the tiny baby phase of life. Not that I would choose to have more, though. I've been letting a lot of baby stuff go, but for some reason this was more symbolic. How silly is that, I didn't even really use the dang thing! But this is my nemesis.... sentimentality.
I felt the same way about the baby sling. Didn't even use it that much, but when I gave it away I was surprised that I was a little sad and wistful.
And it's not silly at all! We use symbols at all our significant stages of life: baby rattles, diplomas, wedding rings, etc. The co-sleeper was your particular symbolic object. You can honor and respect your sentimental feelings. Sadness isn't bad, it's a logical response. Honor it and it can exist along with other feelings.
Sorry to veer off topic!